The primary aim of surge arresters in power distribution lines is to protect lines and equipment from the voltage induced by nearby lightning strokes. For the further improvement of the power system, protection of distribution lines against direct lightning strokes needs to be investigated. One of the effective measures against direct lightning strokes is to increase the number of arresters. However, if the surge current is too large, some surge arresters absorb a large amount of energy in excess of their capability and may be broken, leading to a line fault. To evaluate the protective effect of the surge arresters against direct lightning strokes to overhead ground wire, the authors measured the voltage across arresters and the energy absorbed by surge arresters using a full scale model line and a 12MV inpulse generator. The results were compared with simulation results by EMTP. There have been no previous studies making a comparison of this kind.

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We suggest the superconducting rotary transformer, which is constructed from a synchronous motor combined with a generator. Compared with a usual static transformer, this one has collateral features of (1) improving the power factor, (2) obstructing the harmonic current, (3) avoiding the sudden stoppage power. Therefore it can operate not only as a transformer but also as a filter and a phase modifier on the condition by using superconducting rotary machine.
In this paper, we made a magnet type miniature model with coreless winding of conductor to examine the fundamental characteristics. Though this model had a problem of instability to changing of load, the experimental results provide a confirmation of three characteristics described above.
When the superconducting rotary machines will be practical, this transformer become possible to operate as multifunction power apparatus.

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Improvement of power system stability is the most important issue of generator excitation control. The combination of thyrister excitation with ΔP-PSS is usually applied to recently installed very large turbogenerators. This PSS is not always effective for wide-range operation of generators or for various changes of power system line impedance. The application of the optimal control theory, especially the multi-feedback signal resulting from the Riccati equation has been studied and reported by many papers. These papers, however, do not clearly describe how to set performance index values, and the studies reported in them also use inadequate signals. This paper presents a method to set the performance index values to make adequate feedback signals for selecting actual control on the application of Riccati equation. Also, the deve-loped PSS using multi-feedback signals is applied to actual very large turbo-generators after confirming

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This paper proposes a forecasting method of short-term peak load using a 3-layer neural network of locally active units. Each unit in the hidden layer of the neural network is activated only by input vectors in a bounded domain of vector space. This characteristic enables additional learning. Furthermore, it is supposed to provide the network structure information which would help improving the forecasting accuracy. In simulations, the neural network is applied to daily peak load forecasting in summer. The results show that the proposed method is superior to a conventional neural network with back propagation algorithm. In order to make the best use of the neural network, an error oriented method of parameter modification is also examined.

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Application of controlled series capacitors with antiparallel thyristor combination across the capacitor segment is one of new and promising countermeasures for enhancement of power system stability. The line reactance can be directly controlled by the controlled series capacitors, hence, it is very effective to apply the capacitors for damping power system disturbances.
In this paper, we describe the following.
(1) The design method of control system for the segmented series capacitors with thyristor switches is proposed.
(2)The effectiveness of the controlled capacitors proposed for power system stabilization is shown with the results of EMTP analysis.
(3) The possibilty of zero passing missing phenomena of capacitor voltage is indicated by EMTP simulation, and a countermeasure is proposed in order to avoid the phenomena. The effects of the countermeasure is indicated by the results of EMTP simulation.

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This paper develops a new algorithm for the large scale and long term thermal unit maintenance scheduling problems including maintenance classes. This method is based on the genetic algorithm (GA) combined with the simulated annealing Method (SA). The method takes maintenance class into consideration and minimizes the weighted sum of costs and variance of reserve powers. The proposed method presents a new genetic operation that finds the local optimum faster than the simple genetic algorithm and introduces efficient encoding/ decoding technique. The Boltzmann's acceptance probability of simulated annealing method is included in the algorithm as a criterion for the survival of individuals during the evolution process.
The aim of this study is to reduce the computing time of simulated annealing based methods and make the solution be more accurate than that of the simple genetic algorithm. Numerical results on real scale thermal unit maintenance scheduling which covered several consecutive years are demonstrated, and the scheduling results are compared with those through the simple genetic algorithm or through the simulated annealing method.

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As the power generation planning has become more complex in recent years due to energy resource and environmental constraints as well as current deregulation in the electric power industry, the more detailed long term generation planning, such as monthly planning, taken into account those circumstances must be developed based on the composite view point. In this long term generation planning, the fuel consumption schedule under the given total amount of fuel supply is very important and the computatively effective thermal unit commitment method is also required for the long term operation.
Assuming that an total amount of fuel is allocated for the group of thermal units which use the same kind of fuel, we propose an monthly unit commitment scheduling method with fuel constraint in this paper. The proposed method is basically an extended version of the iterative optimization algorithm based weekly unit commitment method developed by authors earlier and includes algorithms being able to deal with the fuel constraint imposed on the group of thermal units.
Through numerical examples, we show the meaning of the development of monthly unit commitment method and the usefulness of the proposed method in terms of computing burden and flexibility.

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The main transformer's tap changer, which maintains the voltage and reactive power at a constant level on the secondary side of the transformer, must be inspected after a certain number of operations.
This paper outlines a new voltage and reactive power control system that prevents unnecessary tap changing of main transformers by predicting the near-future voltage and reactive power. Control equipment that incorporates this control algorithm has been manufactured for and applied in the Noshiro Thermal Power Plant.

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The paper studied charge accumulation on a supporting insulator and the behaviour of metallic particles which should be especially taken into consideration for DC-GIS. In the studies, we tried to observe the phenomena which happen in a large-scale apparatus, using a 125kV and a 500kV DC-GIS bus model. Distributions of charge accumulation on a cone-type spacer were measured by static probes, changing the magnitude, polarity and duration of dc voltages. Based on the test results, the mechanisms of charge accumulation were proposed. The cone-type spacer designed for DC-GIS showed excellent insulating properties even at the polarity reversal test. We also conducted the calculation studies for the worst case of charge accumulation that a metallic particle standing on the inside of the sheath works as an emission source of the charges. The analysis showed that the electric stresses along the spacer were below the flashover level even at the worst case. In the latter of the paper, the behaviour of the metallic particles placed on a dielectric coated sheath were studied. The studies revealed that the dielectric coating could make the particles inactive up to high electric stress.

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Insulation Level (LIWV 2, 250kV) of 1, 000kV gas insulated switchgear will be adopted by installing high performance 1, 000kV surge arresters. But, when Temporary Over Voltage (TOV) is generated in power systems at line faults and load rejection including Ferranti effects, high performance surge arresters flow discharge current according to the value of TOV. Therefore, it is important to check TOV discharge capability of surge arresters. This paper describes the screening of zinc oxide elements using of thermography (thermal uniformity factor) obtained by an infrared radiation camera, discharge capability of surge arresters for 1, 000kV power system.

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Seismic evaluation of large-capacity thyristor valve is important to design the structure. It is necessary to clarify the criteria of selecting the hypothetic earthquake wave-form in the design since this wave-form affects the equipment design greatly.
The 0.3G three-cycles sinusoidal wave is widely used in Japan to verify the seismic performance of electrical equipment. However, special consideration is needed in designing the structures possessing plural natural frequencies within the predominant frequencies of the earthquake waves. This paper discusses a suitable artificial earthquake waves and a reasonable input wave-form to evaluate the seismic performance of multi-stage structures. Those results are applied to the nearly 1/4 scale model of a large-capacity thyristor valve.

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This paper describes a newly-developed impulse voltage generator (IG). It has the features as follows:
(1) The IG consists of 3 components (a 1.6MV unit, a 1.4MV unit and a base box) for the sake of mounting on 10t trucks, therefore it is easy to move it to a test field.
(2) Capacitors, discharge gaps and a control unit are enclosed in a shelter of fiber reinforced plastics, then it is possible to use the IG in all weathers.
(3) The IG can generate several ten kA when a load is a few ten Ω.
(4) Composition of the circuits for lightning/switching impulse voltage/current tests can be easily done by changing the damping and discharge resistance cassettes. Control and data acquisition are made by a personal computer.
As the application of the IG to an outdoor test, the paper presents the characteristics of resistances of a 77kV substation grounding and a concrete pole when applying a high impulse current. The resistances are dependent on the crest value of the applied current.

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In the converter station of high-voltage DC (HVDC) transmission system. by-pass switch is required to commutate do current to by-pass pair thyristor valves by using its arc voltage when the converter station is get started for operating.
When HVDC gas insulated switchgear (HVDC-GIS) is intended to be installed at the do side of the converter station, SF6 gas circuit breaker is desirable to be used as by-pass switch. But, it has been known that arc voltage of SF6 gas circuit breaker is lower than that of air blast breaker or oil breaker which have been used hitherto as by-pass switches.
This paper showed that SF6 gas circuit breakers were possible to be used as HVDC by-pass switches by describing followings. (I) DC arc voltage characteristics of SF6 gas circuit breaker were measured. (II) Thyristor minimum turn on voltage was measured and it was showed that amplitude of the arc voltages was enough to turn on thyristor valve. (III) Commutation tests with do current 2800A were carried out with a test circuit which simulated the characteristics of anode reactor and thyristor forward voltage drop. the test results showed that a de 2800A current was successfully commutated by the SF6 gas circuit breaker. (IV) Calculations were conducted by simulating the DC current commutation processes and the calculation results agreed well with the test results.

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Charged particle losses on wall surfaces in non-equilibrium MHD power generation plasmas are examined numerically, taking account of the effective flux toward the wall and the surface recombination on it. Electron temperature and density in the bulk plasma are found to be decreased owing to the losses, in particular under the conditions of low bulk electron temperature, low seed fraction and narrow generator channel. Input seed fraction can be reduced by possible sticking of seed material to a cold wall, which is attributed to the flux of seed ions toward the wall. The results obtained suggest that the charged particle losses should be taken into account in the estimate of the performance of MHD generators with narrow channels.

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For the electrical insulation of high voltage apparatus we developed a highly efficient electric field optimization technique using neural network (NN). Once the NN learns the relationship between inputs and outputs, the developed optimization algorithm easily allows to get a target solution. The procedure is as follows: (1)we provide learning patterns and calculate learning data, such as electric field strength, electrode curvature, gap length and coordinate. (2)The NN learns teaching data. (3) Inputting optimum value of electric field strength to NN allows to get an optimum electrode contour. During the optimization process, we proposed an algorithm to determine the optimum field value from the electric field distribution on learning contours. As a result, it took only 12 seconds to reach the optimum contour by personal computer. Consequently, the proposed method in this paper realized the highly efficient electric field optimization by NN.

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